It’s hard to judge without actually seeing the piece.
The place where you may have a problem is in your sprueing —
either too thin or not enough. When I’m doing a piece around 25mm
diameter I typically have 3 or 4 sprues coming back to the main
sprue. These sprues should be at least as thick as piece you are
casting. With a round piece and three sprues attach them to the rim
around 120 degrees apart with one more or less directly above the
sprue base. Remember the molten metal doesn’t like to flow backwards
so visualise the flow of metal when you arrange the sprueing pattern.
It’s better to have too many sprues than not enough. They might be a
right royal pain cut off and finish back but is better than losing a
day from a spoilt casting.
Other problems may include the either the metal or the flask is not
hot enough. The metal should fluid and free running and the flask